Nanofluidics: Theory and Simulations

The lab-on-a-chip concept is a strong driving force for studying liquid motion at small scales. The Department is engaged in exploring the limits of miniaturizability of such chips due to thermal fluctuations and to study their performance. In this context at the nanoscale bulk hydrodynamic equations become invalid and phenomena which are irrelevant on the micronscale or larger, or which can be summarily incorporated in terms of boundary conditions, become important. Among these features are long-ranged molecular interactions, such as dispersion forces, thermal fluctuations, hydrodynamic slip, segregation of mixtures and solutions at walls, and electrical double layers. By using continuum interface dynamics, stochastic thin film equations, mesoscopic hydrodynamics, time-dependent density functional theory, kinetic Monte Carlo simulations, Brownian dynamics, and molecular dynamics simulations, significant progress has been achieved.

Example: Droplets at topographic steps

The long-range van der Waals interactions acting in such systems induce lateral forces on nano-droplets in the vicinity of chemical and topographical steps even if the three-phase contact line of the droplets does not touch the step.

A colloid (green) is held in a laser focus (optical tweezer; red) and dragged through a solution of polymer coils (blue). The polymer coils accumulate infront of the colloid and increase the drag force above the value expected from the Stokes equation.